This invention relates to a structural steel that is subjected to cold forging, either as-rolled or after rolling and annealing, and a method of producing such a steel.
Steels used for structural members are passed through various forming processes in order to impart required properties to them. Radio-frequency hardening, for hardening the surface layer, is one of these processes. Since such structural members are required to have only a high surface layer hardness, in most cases, an increase in the number of processes results in an increase of the cost of production, and this has been one of the problems in the past. Since as-rolled materials of the conventional structural steels have a low cooling rate, they have a ferrite-pearlite structure in most cases. However, their surface layer hardness is low and never reaches the level achievable by radio-frequency hardening. More often than not, the surface layer hardness is lower than the internal hardness due to the influence of decarburization, and so forth. Though ordinary members need not always have a maximum hardness corresponding to the C (carbon) content brought forth by radio-frequency hardening, it is undeniable that some of the members are required to have a hardness higher than that of the annealed materials. Therefore, the provision of steels having, as-rolled, a higher surface layer hardness than the internal hardness has been another problem.
When complicated shapes are required, the steel materials are passed through forging and cutting processes. Because hot forging needs heating and has a low forming accuracy, cold forging, having higher forming accuracy, has been preferred. Nonetheless, conventional as-rolled materials are not suitable for cold forging because the hardness is too high. Ordinary steels for cold forging are generally softened by spheroidizing cementite. The annealing time is extremely long and is as much as about 20 hours.
The prior art references such as Japanese Unexamined Patent Publication (Kokai) No. 3-140411 describe that cold formability and cuttability of even a steel having a carbon content equivalent to the level of carbon steels for cold forging can be improved by graphitizing carbon and converting the steel structure to a ferrite-graphite dual phase. However, annealing for a long time is necessary to achieve such a structure, and the problems of production efficiency and production cost are left unsolved. In other words, the problem of shortening the annealing time is yet to be solved.
In order to reduce the graphitization annealing time, a technique has been suggested which adds B and uses BN as precipitation nuclei. However, when such a specific precipitate is used, a temperature-retaining process, in the BN precipitation temperature range, is necessary before annealing is conducted, and an additional annealing process becomes necessary. If this heat-treatment is conducted conjointly by rolling or hot forging, temperature control must be conducted extremely strictly until annealing, and this is virtually impossible.
In other words, the precipitation temperature of BN is believed to be from about 850 to about 900xc2x0 C., but rolling and hot forging are actually carried out at a temperature higher than 1,000xc2x0 C. in many cases. Therefore, in order to use such a graphite-containing steel for cold forging, rolling and hot forging, as prior processes, must be conducted at a temperature below 1,000xc2x0 C. Hot forming at such a temperature lowers the service life of tools such as rolls and punches. The increase of the number of limitations on the processes leads to the drop of production efficiency, and must be therefore avoided to restrict the increase of the production cost. From the aspects of steel making and hot forging, as a prior process to cold forging, steel materials that do not need strict temperature control and can be annealed and softened within a short time have been required.
Japanese Unexamined Patent Publication (Kokai) No. 2-111842 teaches shortening the annealing time by restricting the graphite content within a short time. However, this technology does not provide a fundamental solution because cold forgeability and cuttability are deteriorated in proportion to the amount of cementite that remains in the steel materials as a result of suppression of the graphite content.
As described above, the conventional as-rolled materials are not entirely satisfactory because their surface layer hardness is not sufficient when they are used as such, but it is too high when they are subjected to cold forging and cutting. From the viewpoint of production, on the other hand, there is the fundamental problem that the steels should preferably be produced collectively by reducing the number of their kinds in order to reduce the cost of production. Therefore, it has been desired that the as-rolled materials have a sufficient surface hardness, the annealing time can be shortened when the as-rolled materials are subjected to cold forging, and they can exhibit excellent cold forgeability after annealing.
When strength is also further required, it may be possible, in principle, to add those elements which do not impede graphitization for improving hardenability but can improve hardenability. Particularly when the surface hardness by radio-frequency hardening is necessary, hardenability becomes more different problem because of increase the thickness of the hardened layer. However, since ordinary hardenability improving elements such as Cr, Mn, Mo, etc, hinder graphitization, the amounts of addition are limited. When the graphitization annealing time is shortened by forming BN, B cannot be used as the hardenability improving element, and the hardening depth cannot be sufficiently secured, either.
Under the above-described condition, a steel which makes it possible to reduce the annealing time, and is excellent in cold forgeability after annealing, hardenability and cuttability, has been required.
It is an object of the present invention to provide a steel that has, as-rolled, excellent surface hardness, by regulating the chemical components of the steel and its microstructure, and can impart excellent cold forgeability within an extremely short softening/annealing time before cold forging and cutting, and to provide a method of producing the steel.
It is another object of the present invention to provide a steel, for cold forging after annealing, that can shorten the annealing time, by regulating the chemical components of the steel, is excellent in cold formability and cuttability after annealing and has excellent strength and toughness after hardening and tempering.
To accomplish these objects, the present invention provides the following inventions.
(1) The first invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, that contains, in terms of wt %, C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%, S: not greater than 0.500%, sol. N: being limited to not greater than 0.005%, and the balance consisting of Fe and unavoidable impurities, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120xc3x97(C %) % (with the maximum being not greater than 100%), and the outermost surface layer hardness is at least 450xc3x97(C %)+90 in terms of the Vickers hardness HV.
(2) The second invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains at least one of Cr: 0.01 to 0.70% and Mo: 0.05 to 0.50%, in addition to the chemical components of the first invention (1) described above, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120xc3x97(C %) %, and the outermost surface layer hardness is at least 450xc3x97(C %)+90 in terms of the Vickers hardness HV.
(3) The third invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains at least one of Ti: 0.01 to 0.20%, V: 0.05 to 0.50%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.30% and Al: 0.001 to 0.050% in addition to the chemical components of the paragraph (1) or (2) described above, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio on microscope plate/microscope plate area) is not grater than 120xc3x97(C %) %, and the outermost surface layer hardness is at least 450xc3x97(C %)+90 in terms of the Vickers hardness Hv.
(4) The fourth invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains B: 0.0001 to 0.0060% in addition to the chemical components of any of the paragraphs (1) to (3), wherein a pearlite ratio in the steel structure (pearlite occupying area ratio on microscope plate/microscope plate area) is not greater than 120xc3x97(C %) %, and the outermost layer surface hardness is at least 450xc3x97(C%)+90 in terms of the Vickers hardness Hv.
(5) The fifth invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains Pb: 0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to 0.100%, Se: 0.01 to 0.50% and Bi: 0.01 to 0.50% in addition to the chemical components of any of the paragraphs (1) to (4), wherein a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120xc3x97(C %) %, and the outermost layer hardness is at least 450xc3x97(C %)+90 in terms of the Vickers hardness Hv.
(6) The sixth invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains Mg: 0.0005 to 0.0200% in addition to said chemical components according to any of claims 1 through 6, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio on microscope plate/microscope plate area) is not greater than 120xc3x97(C %) %, and the outermost surface layer hardness is at least 450xc3x97(C %)+90 in terms of the Vickers hardness HV.
(7) The seventh invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains, in terms of wt %, C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%, S: not greater than 0.500, sol. N: being limited to not greater than 0.005% and the balance consisting of Fe and unavoidable impurities, and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10xc3x97(C %)⅓ xcexcm and the maximum crystal grain diameter is not greater than 20 xcexcm.
(8) The eighth invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains at least one of Cr: 0.01 to 0.70% and Mo: 0.05 to 0.50%, and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10xc3x97(C %)⅓ xcexcm , and a maximum crystal grain diameter is not greater than 20 xcexcm.
(9) The ninth invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains at least one of Ti: 0.01 to 0.20%, V: 0.05 to 0.50%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.30% and Al: 0.001 to 0.050% in addition to the chemical components described in the paragraph (7) or (8), and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10xc3x97(C %)⅓ xcexcm, and a maximum crystal grain diameter is not greater than 20 xcexcm.
(10) The tenth invention provides a steel for cold forging, which contains B: 0.0001 to 0.0060% in addition to the chemical components of any of the paragraphs (7) to (9), and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10xc3x97(C %)⅓ xcexcm and a maximum crystal grain diameter is not greater than 20 xcexcm.
(11) The eleventh invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains Pb: 0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to 0.100%, Se: 0.01 to 0.50% and Bi: 0.01 to 0.50% in addition to the chemical components of any of the paragraphs (7) to (10), and has a structure wherein a ratio of a graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of graphite is not greater than 10xc3x97(C %)⅓ xcexcm, and a maximum crystal grain diameter is not greater than 20 xcexcm.
(12) The twelfth invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains Mg: 0.0005 to 0.0200% in addition to the chemical components of any of the paragraphs (7) to (11), and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10xc3x97(C %)⅓ xcexcm, and a maximum crystal grain diameter is not greater than 20 xcexcm.
(13) A method of producing a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which comprises the steps of rolling the steel having the chemical components of any of the paragraphs (1) to (6) described above in an austenite temperature zone or in an austenite-ferrite dual phase zone so that a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120xc3x97(C %) % and the outermost surface layer hardness is at least 450xc3x97(C %)+90 in terms of the Vickers hardness Hv; rapidly cooling the steel immediately after the finish of rolling at a rate of at least 1xc2x0 C./s; and controlling a recuperative temperature to 650xc2x0 C. or below.